The present invention relates to a liquid crystal display device driven by switching whole screen display and partial screen display as required, for achieving low voltage driving of a portable telephone, etc.
Recently, liquid crystal display (LCD) devices have been used for various purposes, for instance, as displays for use in portable information terminals, for the reason that they are thin in thickness and light in weight.
LCD devices are light-receiving-type elements that execute display by not emitting light per se but varying intensities of light permeated, and can be driven with a low effective voltage of about several volts. Therefore, by rendering an LCD device a reflection-type LCD device that executes display with external light reflected by a reflection plate provided beneath the device, it becomes a display element of extremely low power consumption. Further, by rendering such a reflection-type LCD device an STN-type LCD device driven by time division driving, the panel structure is simplified while low power consumption is enabled, and further low price is realized.
Here, time division driving is briefly explained. The time division driving is a driving method in which a selection waveform is applied to each scanning electrode line by line, while the same scanning is repeated when the selection waveform has been applied to all the scanning electrodes. Time while such a scanning is carried out is a frame period, and a frequency thereof is a frame frequency. Furthermore, a ratio of a time of selecting each scanning electrode (time necessary for applying the selection waveform to each scanning electrode) to a frame period is called a duty ratio.
In the time division driving, an electric field is applied not only to ON pixels but also to OFF pixels. Therefore, a threshold value characteristic becomes an essential condition determining electro-optical characteristic of the LCD device.
A waveform useful for control of a display state for the time division driving is applied during only a certain period of time determined according to the duty ratio, while a waveform having nothing to do with the control of a display state is applied for most of the rest of time. Liquid crystal responds to an applied waveform during the non-selection time, and to suppress a decrease of a display contrast (cross-talk phenomenon), it is necessary to make the effective voltage of the application waveform during non-selection time constant. This driving method in which the display state is made constant is called as amplitude selective addressing scheme. Incidentally, the term of the effective voltage means a square mean voltage of applied voltages in one frame period.
An LCD device performing the aforementioned time division driving, whose effective voltage upon driving is low, consumes very low power, as compared with an active-matrix-type LCD device having a high effective voltage upon driving. Therefore, it draws attention regarding application for carrying purpose, and many attempts for lower power consumption have been made by lowering a voltage upon driving.
To lower the power consumption of the LCD device, an approach of lowering the effective voltage of the LCD panel section is usually taken. In the foregoing approach, attempts for increasing a liquid crystal dielectric constant for lowering voltages upon driving are made day and night.
Furthermore, not only regarding the LCD panel section side, but also regarding the LCD driver side, attempts to decrease power consumption has been made: for instance, the Japanese Publication for Laid-Open Patent Application No. 97219/1992 (Tokukaihei 4-97219 [Publication Date: Mar. 30, 1992]), and the Japanese Publication for Laid-Open Patent Application No. 113314/1992 (Tokukaihei 4-113314 [Date of Publication: Apr. 14, 1992]) teach a driving method whereby contrast is improved by lowering the bias ratio in the time division driving.
Furthermore, regarding the liquid crystal driver, as a technique for more effectively lowering power consumption, like a technique applied to portable telephones or the like, a driving technique (partial driving) is applied in which a maximum amplitude value of a voltage waveform applied to a driver is lowered by switching the whole screen display to the partial screen display when the device is in the stand-by state. Such a partial driving technique is described in the Japanese Publications for Laid-Open Patent Applications No. 149184/1994, 207438/1998 (Tokukaihei 6-149184 [Date of Publication: May 27, 1994], Tokukaihei 10-207438 [Date of Publication: Aug. 7, 1998]).
The Japanese Publication for Laid-Open Patent Application No. 6-149184 (Tokukaihei 149184/1994) teaches a method for switching the whole screen display and the partial screen display, for partial driving, and more specifically, an LCD device that includes, in one and same LCD panel, portions displayed by driving at high duty and portions displayed by driving at low duty, and by switching the high-duty driving and the low-duty driving the LCD panel is formed smaller in size and that costs low. However, this publication teaches nothing about the driving conditions such as the setting of bias or frequency.
On the other hand, the Japanese Publication for Laid-Open Patent Application No. 10-207438 teaches driving conditions such as setting of bias for the partial driving. More specifically, in the amplitude selective addressing scheme, a ratio of an application voltage effective value of an ON voltage (hereinafter referred to as effective value of an ON voltage) to an application voltage effective value of an OFF voltage (hereinafter referred to as effective value of an OFF voltage) can be set as great as possible, and a bias ratio is set so as to be in a driver voltage-resistant range. However, the foregoing publication does not show anything about the setting of a frequency.
To decrease the effective voltage of the LCD device, as described above, it is indispensable to increase the dielectric constant of liquid crystal. However, ionic impurities taken into liquid crystal in the production process increase as the dielectric constant of liquid crystal increases. This raises a drawback in that reliability lowers, for instance, display defects are produced while turned ON.
On the other hand, as described above, as a driving voltage of a driver lowers by partial driving, low power consumption is realized in the LCD device as a whole. In an STN-type LCD device, in partial driving, a bias ratio is determined by the amplitude selective addressing method so that the driving voltage during ON time should be in a range of pressure resistance of the driver upon partial driving as well as that a ratio of an effective value of the ON voltage to an effective value of the OFF voltage can be as great as possible. Here, as shown in FIG. 4, when the effective value of the OFF voltage of partial screen display of the liquid crystal is made to coincide with the effective value of the OFF voltage of the whole screen display (the point of coincidence is indicated with A in the figure), an effective value of the ON voltage in the partial screen display (indicated with C in the figure) exceeds an effective value of the ON voltage of the whole screen display (denoted with B in the figure). Therefore, the power consumption of the liquid crystal display panel section in the partial image display increases as compared with that of the whole screen display.
Furthermore, an increase in the effective voltage value upon partial screen display causes the pulse voltage load applied to liquid crystal molecules to increase. Therefore, as shown in FIG. 5, display defects are produced at edges of the display area of the LCD panel, during partial screen display.
Furthermore, by making the effective value of the ON voltage of the partial screen display exceed the effective value of the ON voltage of the whole screen display, the voltage shifts from an optimal voltage, thereby causing color tones to lower.
An object of the present invention is to provide an LCD device that executes a driving operation (partial driving) by switching whole screen display and partial screen display as required, and that is further arranged so as to have driving conditions for setting the ON voltage and the OFF voltage at the partial screen display time, whereby optical characteristics such as color tone and contrast can be improved, no display defect is produced, and reliability is improved.
To achieve the foregoing object, an LCD device in accordance with the present invention is an LCD device that switches whole screen display and partial screen display as required, to perform time division driving, and the liquid crystal display device is characterized by comprising:
a driving circuit executing a driving operation in a manner such that an applied voltage effective value of an OFF voltage at the partial screen display time and an applied voltage effective value of an OFF voltage at the whole screen display time should substantially coincide with each other, as well as an applied voltage effective value of an ON voltage at the partial screen display time and an applied voltage effective value of an ON voltage at the whole screen display time should substantially coincide with each other.
As described above, in the case of an LCD device driven by switching the whole screen display and the partial screen display as required (partial driving), low power consumption of the device as a whole can be realized, since the driving voltage of the driver lowers. In addition to this, in the case of an LCD device of the present invention that carries out time division driving under the foregoing set conditions, since the applied voltage effective value of the OFF voltage at the partial screen display time and the applied voltage effective value of the OFF voltage at the whole screen display time should substantially coincide with each other, as well as the applied voltage effective value of the ON voltage at the partial screen display time and the applied voltage effective value of the ON voltage at the whole screen display time should substantially coincide with each other, no difference in optical characteristics is produced between the whole screen display while the OFF voltage is applied and the partial screen display while the OFF voltage is applied, as well as between the whole screen display while the ON voltage is applied and the partial screen display while the ON voltage is applied.
Therefore, even in the partial screen display at which conventionally display defects tend to occur as compared with the whole screen display, the display performance can be improved, to have equal levels to those at the whole screen display time, regarding the color tone, contrast, and even display defect level.
For a fuller understanding of the nature and advantages of the invention, reference should be made to the ensuing detailed description taken in conjunction with the accompanying drawings.